JPH04578B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to a protection method when a persistent current switch in a superconducting magnet device is quenched.

[Prior art and its problems]

Superconducting magnets using superconducting wires can have high current density, so they can generate a strong magnetic field in a wide space with a compact magnet and with relatively low power. The superconducting wires currently in practical use are composite superconducting wires in which fine wires of superconducting material are embedded in high-purity metals such as copper and aluminum (these are called stabilizing materials). Copper and aluminum are used at liquid _helium temperatures (approx.
Even at 4.2K), it does not become superconducting. However, these stabilizing materials have a very low resistivity at a temperature of 4.2K, so if the superconductor loses its superconducting state for some reason and enters a high resistance state (the so-called quench phenomenon), it will flow through the superconductor. The current is guided to the bypass circuit to reduce Joule heat generation and prevent burnout of the superconducting wire.

The ratio of the stabilizing material to the superconductor (generally referred to as the copper ratio) in the composite superconducting wire depends on the cooling condition of the manufactured magnet during use, the size of the magnet, and the operating method. In general, superconducting magnets with a large stored energy have a large copper ratio in the composite superconducting wire and a low current density. Therefore, it is most preferable to operate a superconducting magnet with a large significant energy at a current density as high as possible.

When the magnet is quenched, the stored magnetic energy is consumed as the aforementioned Joule heat generation from the stabilizing material, and this Joule heat generation increases the temperature of the magnet. This maximum temperature T _M is expressed as T _M ∝J _O ² ·τ (1). Here, J _O is the current density in the stabilizing material, τ is the decay time constant, and τ≡L/R (L is the inductance of the magnet, and R is the resistance value of the circuit. In this case, the resistance value generated in the superconducting magnet is dominant. target). This maximum temperature T _M is usually designed to be about 300K for small magnets to protect the magnet, 200 to 100K for medium magnets, and about 70K for large magnets that allow quenching. The circuit resistance is due to the resistance of the coil itself, and as shown in Figure 5, after detecting the quench of the magnet 1, the circuit breaker 4 is opened to disconnect the magnet 1 from the power source 5, and a protective resistor 2 is inserted in series with the magnet 1. A part of the stored energy of magnet 1 is transferred to the room temperature range.
A protection method is often used that can suppress the maximum temperature of the magnet 1 after quenching by dissipating it with a protective resistor installed in (A), or provide a magnet operated at a higher current density. Note that 3 in FIG. 5 indicates a current supply lead for supplying an excitation current to the magnet 1, and (L) indicates an extremely low temperature region.

Superconducting magnets operate in so-called persistent current mode, where both terminals of the superconducting magnet are short-circuited using a persistent current switch using superconducting wire, and the current supply lead is pulled out during normal use except for excitation and demagnetization of the magnet. It is normal to If the persistent current switch quenches and enters a high resistance state in the persistent current mode, the energy stored in the magnet will be concentrated in the persistent current switch, leading to burnout of the switch. Therefore, it is known to use a protective resistor 2 connected in parallel with a switch 6 as shown in FIG. In this case, the value of the resistance value R2 of the protective resistor ₂ needs to be sufficiently smaller than the normal conduction resistance value R _S when the switch 6 is in the OFF state, but these values should be determined based on the stored energy of the magnet and practical excitation. Determined based on time. Furthermore, the protective resistor 2 must be placed sufficiently away from the magnet, for example, in the refrigerant gas space L2, and configured so as not to interfere with magnet excitation. The magnet and switch shall be installed in the refrigerant liquid area.

When designing a persistent current switch and its protective resistor, let R _S be the resistance value when the switch is OFF, that is, during normal conduction, W・R _S ∫〓 ^m _4.2 C _S dθ≧R _S ／R ₂ +R _S・E …(2) must be satisfied. Here, W: Mass per 1Ω of the material that makes up the switch [Kg/
Ω], ∫〓 ^m _4.2 C _S dθ: Enthalpy increase when the switch reaches the maximum temperature θm (K) [J/Kg], E:
This is the stored energy (J) of a superconducting magnet. In other words, the energy that can be absorbed by the switch and the maximum temperature θM of the switch must satisfy equation (2), and θm is
It is desirable to exceed 300K. As the stored energy increases, the problem arises that the persistent current switch must also be larger and the heat-resistant load must be increased, and the larger switch also causes the problem of poorer ON-OFF response.

[Purpose of the invention]

This invention is designed to protect the persistent current switch sufficiently safely without increasing the size of the switch when a superconducting magnet with large magnetic stored energy is operated in persistent current mode. The purpose of this invention is to provide a superconducting magnet device.

[Key points of the invention]

In a superconducting magnet that is operated using a persistent current switch, the present invention provides superconducting by installing the protective resistor of the switch very close to the superconducting magnet, and by quenching the persistent current switch and generating heat from the current flowing through the protective resistor. The idea is to heat the magnet and quench the magnet itself, thereby consuming the stored energy inside the magnet, thereby suppressing energy consumption in the switch, and thereby miniaturizing the persistent current switch.

[Embodiments of the invention]

Figure 1 is an electrical circuit diagram of a superconducting device showing an embodiment of the present invention, and Figures 5 and 6 are conventional electrical circuit diagrams.
Components that are the same as those in the figures are given the same reference numerals and explanations will be omitted. The protective resistor 2 of the persistent current switch 6 is
It is connected in series with a diode 7 provided to prevent current from flowing through the protective resistor 2 due to the voltage generated during demagnetization, and this circuit is electrically connected in parallel to the superconducting magnet 1 and the persistent current switch 6, respectively. The superconducting magnet 1 is connected to the superconducting magnet 1 and located close to the superconducting magnet 1 . In this case, since the voltage during excitation and demagnetization of the superconducting magnet is extremely small, no current flows through the protective resistor due to the forward breakdown voltage (about several volts) of the diode 7. The reason why the diodes are connected in antiparallel is to be able to handle either polarity (+ or -) of the circuit. Due to the combined resistance value R _2S (R _2S = 1/ _RS + 1/R ₂ ) of the resistance value R _S generated by the quenching of the persistent current switch 6 and the resistance value R ₂ of the protective resistor 2, a voltage is generated between the terminals af. The induced diode 7 turns on, and in persistent current mode
Most of the current flowing through bcde flows through the ac df circuit, which includes two protective resistor circuits. Therefore, Joule heat generation occurs in the protective resistor 2, and the superconducting magnet 1 in the vicinity is heated, leading to quenching.

Fig. 2 is a vertical cross-sectional view showing a specific method of installing the protective resistor of a persistent current switch, in which the protective resistor of the persistent current switch is tightly attached to the outer peripheral surface of the superconducting winding 1a wound in a solenoid shape around the winding frame 8 having flanges at both ends of the cylindrical body. The protective resistor 2 is then wound through the persistent current switch 6. The protective resistor 2 is made of copper, stainless steel, or the like, but in this case, the protective resistor layers 2a and 2b are connected in opposite directions so that the currents flow in the layers 2a and 2b in opposite directions so that the protective resistor has no inductance. Furthermore, the protective resistor installed closely on the outside of the superconducting winding has the advantage of simultaneously suppressing electromagnetic force.

FIG. 3 is a perspective view showing a state in which cooling holes 9 are provided in the protective resistor layers 2a and 2b of FIG. 2 to promote cooling of the superconducting winding 1a during normal operation.

Figure 4 shows the switch's protective resistor connected to superconducting winding 1.
It is a longitudinal cross-sectional view showing another example installed inside a.

〔Effect of the invention〕

According to this invention, the protective resistor against quenching of the persistent current switch is arranged in close proximity to the superconducting magnet winding, so that after the switch is quenched, the protective resistor is energized, and the resulting heat generation also quenches the superconducting magnet. Therefore, the released magnetic energy can be dispersed and consumed. Therefore, the heat resistance capacity of the persistent current switch can be reduced. Therefore, even with superconducting magnets that have a large amount of stored energy, a small persistent current switch can be used.In particular, thermal persistent current switches can improve ON-OFF response and can be manufactured using fewer superconducting wires. Therefore, magnetic stability is also improved.

Furthermore, by dispersing the energy consumption parts, the protective resistor itself can be made smaller, and the quench caused by the protective resistor tends to spread over the entire magnet in a short period of time, rather than locally quenching the magnet. It is possible to prevent local temperature rise of the magnet.

[Brief explanation of drawings]

FIG. 1 is an electric circuit diagram of a superconducting magnet device according to the present invention, FIG. 2 is a vertical cross-sectional view of a superconducting magnet with a protective resistor installed on the outside as a first embodiment of the present invention, and FIG. 3 is a diagram of a superconducting magnet according to the present invention. As a second embodiment, the second
FIG. 4 is a vertical cross-sectional view of a superconducting magnet with a protective resistor installed inside as a third embodiment of the present invention; FIG. FIG. 6 is a diagram showing a protection circuit of a conventional superconducting magnet. 1: Superconducting magnet, 2, 2a, 2b: Protective resistor, 6: Persistent current switch, 7: Diode,
8: Winding frame, 9: Cooling hole.

Claims (1)

[Claims] 1. In a superconducting magnet operated using a persistent current switch; a protective resistor against quenching of the persistent current switch is provided in close contact with the superconducting winding, and the protective resistor is electrically connected in antiparallel. A superconducting magnet device characterized in that the superconducting magnet device is configured to be connected in series with a diode and connected in parallel with the superconducting winding. 2. In the superconducting magnet device according to claim 1; a superconducting winding is wound around a winding frame having flanges at both ends of a cylindrical body, and a protective resistor divided into an even number of layers is wound around the outer periphery of the winding frame. A superconducting magnet device characterized by comprising: 3. In the superconducting magnet device according to claim 1; a protective resistor divided into an even number of layers is wound around a winding frame having flanges at both ends of a cylindrical body, and a superconducting winding is wound around the outer periphery of the winding frame. A superconducting magnet device characterized by comprising: 4. In the superconducting magnet device according to claims 2 and 3; the protective resistor is divided into an even number of layers and is wound so that the current flowing through each layer is connected in the opposite direction. A superconducting magnet device characterized by: 5. The superconducting magnet device according to claim 2, wherein the protective resistor is divided into an even number of layers and wound, and has a plurality of cooling holes penetrating each layer.